Adjusting the offset frequency and the center frequency will adapt the decoder input to a receiver IF output.
The minimum offset frequency resolution available with the decoder is 1 Hz. The effective center frequency is the sum of the offset frequency and the center frequency. The function is similar to the mixing of the signal frequency and BFO of a receiver.
An exception is the FFSK demodulator for direct frequency modulation: in this case the indicated offset frequency is equal to the effective center frequency.
An example for an HF receiver and ARQ-E mode:
Ø Receiver IF 455,000 Hz
Ø Offset 453,300 Hz
Ø Center 1700 Hz
The advantage of this method is that the user only needs to know the center frequency.
Another example for a VHF-UHF receiver and POCSAG mode:
Ø Receiver IF 455,000 Hz
Ø Offset 455,000 Hz
Ø Center 0 Hz
In this case the offset frequency is equal to the center frequency.